Ratios: (gyrofrequency/collision freq), (gyroradius/ device L)
(Te ~ 2 eV, Ar)
15 16 -3 Since nn typically 10 – 10 cm currently, need B > ~ 200 G, so electrons may already be magnetized in some expts.
ions
(Ti ~ .05 eV, Ar)
Since mi >> me, Ti << Te (while σin ~ 10 σen), need larger B to magnetize the ions:
B > 0.5 – 2 T with current nn (argon) dust
(ρd ~ 2g/cc, Ar, Tn ~ 300 K)
Larger B needed as R↑, nn ↑, Td ↑, φ↓, L ↓
e.g., φ ~ - 4 V, R ~ 0.1 µm, P ~ 10 Pa, B >> 30 T
as above, with P ~ 0.13 Pa, B >> 0.4 T Use of small dust having surface plasmon resonance?
• Background: Surface Plasmon (ne wave) at interface
Polarizability of small sphere 2R << λ
light ε ε0
E-field e-cloud resonance
Re ε = - 2 ε0 (Im ε << 1)
surface plasmon - optical, near-UV (Au, Ag) Possible way to see nano-sized dust in plasma
• Absorption and scattering enhanced at SP frequency
• Use of dust with SP
- ‘see’ nanoparticle transport in plasma
- ‘see’ waves, etc. in plasma with magnetized dust (R~ 20 nm, B ~ 2T) Possible diagnostic for dust waves in magnetized dusty plasma
• Nm sized dust could be magnetized Ag ~ 40 nm e.g. Ar, Te ~ 2 eV, B ~ 2 T, P < 3 Pa
ρd < ~ 1 cm, ωcd > νd , F Lorentz > F gravity
• ‘See’ waves by enhanced absorption by dust with SP Compression - dark; rarefaction - light
n ~ 109 cm-3 d Absorption coeff. ~ 0.1 /cm λ ~ 350 nm (Re ε ~ -2)
Rosenberg & Shukla, APL 86, 2005 Theory has been done for dust wave instabilities under the following conditions, but not aware of experiments to test these :
Unmagnetized dust, magnetized ions e.g. Hall current (E x B) driven instabilities drift instabilities
Magnetized dust e.g. electrostatic dust cyclotron instability (higher harmonics too) waves in plasma crystals Possible constraint related to Electrostatic Dust Cyclotron waves
EDC wave: in long λ limit, k┴ρd << 1,
2 2 2 ω ~ ωcd (1+Δ), Δ ~ k┴ csd /2ωcd << 1,
where the dust acoustic speed csd ~ λDiωpd (for Te >> Ti)
-4 9 e.g. Ti ~ 0.2 eV, nd/ni ~ 10 , md/mp ~ 2 x 10 (R ~ 0.1 µm),
B ~ 2 T λ┴ > 7 cm ( λ║ > λ┴) , larger device ? Estimates of B for controlling motion of paramagnetic dust
Induced magnetic moment
Ratio of forces: attractive magnetic dipole-dipole to repulsive Coulomb
B d
Tune grain interactions via FM, using superparamagnetic grains
e.g., φ ~ - 4 V, R ~ 5 µm, d ~ 100 µm, µ ~ 3 , B ~ 200 G , FM/Fe ~ 0.1
as above with R ~ 0.3 µm, FM/Fe ~ 0.1 requires B ~ 5 T
as R ↓ , B ↑ Magnetic packing force: magnetic grains move to regions of max. B
Ratio of forces: magnetic packing to attractive magnetic dipole-dipole
e.g., as in previous example, R ~ 5 µm, d ~ 100 µm, µ ~ 3, B ~ 200 G,
FMP/FM > 1 if ∂B/∂z ~ 6 G/cm
3 Levitate [1] : FMP, grav. force both proportional to R
e.g., µ~ 3, B ~ 1000 G, ∂B/∂z ~ 20 G/cm, ρd ~ 2g/cc
[1] Samsonov et al, NJP, 2003 Some physics issues that could be studied at large B ~ T
Charging: Debye sphere, anisotropic, E x B of electrons (ρe ~ R) ?
Forces: effect of magnetized ions (ρi ~200 µm) on ion drag?
Fusion: how does B affect structure of ablation cloud around large grain?
Waves: observe EDC waves and instabilities?
Crystal: how do magnetized ions & electrons affect structure and waves?
(e.g., ρi ~ lattice spacing, E x B motion in sheath )
Crystal: can crystal form if dust is magnetized (smaller R, lower Zd) ? Some physics issues that could be studied at B < T
(magnetized ions and/or electrons, unmagnetized dust)
Waves: observe dust wave instabilities driven by electron or ion cross –field drifts, including Hall current instabilities, and drift wave instabilties?
Paramag. dust: tune behavior of dust acoustic waves (attractive force along B, repulsive perpendicular to B)?
Paramag. dust: tune spacings and structure in a plasma crystal by changing magnitude and orientation of B?
Paramag. dust: use FM + Fe force to levitate one type of grain, Fe to levitate other type, form binary crystal?